`Norris
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`Patent Number:
`[11]
`[45] Date of Patent:
`
`5,781,150
`Jul. 14, 1998
`
`[54]
`
`[75]
`
`[73]
`
`GPS RELATIVE POSITION DETECTION
`SYSTEM
`
`Inventor:
`
`Elwood G. Norris. Poway. Calif.
`
`Assignee: American Technology Corporation.
`Poway. Calif.
`
`[21]
`1221
`
`Appl. No.:
`542,799
`Oct. 13, 1995
`
`Filed:
`
`Related US. Application Data
`
`[63] Continuation-impart of Ser. No. 377,973, Jan. 25, 1995, Pat.
`No. 5,689,269.
`
`[5 1]
`[5 2]
`[5 3]
`
`[56]
`
`Int. Cl.6 ............................. .. H048 7/185; GOlS 5/02
`US. Cl. ........................................... .. 342/357; 342/419
`Field of Search ................................... .. 342/357. 419;
`455/121
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`3,063,048
`4,021,807
`4,593,273
`4,675,656
`5,021,794
`5,119,504
`5,146,231
`5,172,110
`5,245,314
`5,289,195
`5,307,277
`5,434,789
`5,506,587
`5,539,398
`
`11/1962 Lehan et a1. .
`5/1977 Culpepper et a1. .
`6/1986 Narcisse.
`6/1987 Narcisse.
`6/1991 Lawrence.
`6/1992 Dur’ooraw, III .
`9/1992 Ghaem et a1. ........................ .. 342/419
`12/1992 Tiefengraber.
`9/1993 Kah, Jr. .
`2/1994 lnoue.
`4/1994 l-lirano.
`7/1995 Fraker et a1. .................. .. 364/460
`
`4/1996 Lans . . . . . . . . . . . .
`
`. . . .. 342/357
`
`7/1996 Hall et a1. ............................. .. 340/907
`
`OTHER PUBLICATIONS
`GPS Technology and Opportunities by Clyde Haris and Roy
`Sikorski.
`Utah Meeting Shows Amazing World of Navigation Satel
`lites By Lee Siegel.
`
`A Marriage Made In Orbit: GPS and PCS by Francis X.
`Kane.
`A Sampling Of Global Positioning System Receivers by
`Don Herslrovitz.
`How Mobile Computers Can Help You Find Yourself by
`Gerald Houston.
`
`This ‘Remote’ Shows Its Users Exactly Where Here Is by
`Liz Mullen.
`
`CAR 54. Where Are You? By Michael Puttre.
`
`United States Securities And Exchange Commission Form
`10-K FRO Trimble Navigational Limited
`
`Primary Examiner-Theodore M. Blum
`Attorney, Agent, or Firm—Thorpe. North & Western. LLP
`
`[57]
`
`ABSTRACT
`
`A system of GPS devices which receive civilian GPS signals
`and provide an intuitive graphical interface for displaying
`the relative position of GPS devices in relation to each other.
`the relative position being accurate to several meters and
`de?ned as the distance to. direction of and height variance
`between GPS devices. A ?rst GPS device with the person or
`object to be located transmits its GPS determined location to
`a second GPS device. This second GPS device includes a
`means for receiving the GPS determined position of the ?rst
`GPS device. and also includes means for calculating the
`relative position of the ?rst GPS device relative to the
`second GPS device based on a comparison of the received
`telemetry of the ?rst GPS device and its own GPS deter
`mined position. The relative position of the ?rst device is
`then graphically displayed on an interface of the second GPS
`device in a manner which eliminates the need for a map in
`order to travel to the location of the ?rst GPS device. While
`proliding an interface which displays a relative position of
`the ?rst GPS device. this information remains accurate no
`matter how the orientation of the second GPS device
`changes with respect to a compass,
`
`13 Claims, 8 Drawing Sheets
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`ZTE Exhibit 1005
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`US. Patent
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`Jul. 14, 1998
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`Sheet 1 of 8
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`5,781,150
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`ZTE Exhibit 1005 - 2
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`US. Patent
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`Jul. 14, 1998
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`Sheet 2 of 8
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`5,781,150
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`Jul. 14, 1998
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`5,781,150
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`Jul. 14, 1998
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`Jul. 14, 1998
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`5,781,150
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`Jul. 14, 1998
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`5.781.150
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`1
`GPS RELATIVE POSITION DETECTION
`SYSTEM
`
`RELATED INVENTION
`This patent application is a continuation-in-part of US.
`patent application Ser. No. 08/377973. filed Jan. 25. 1995.
`now US. Pat. No. 5.689.269.
`
`2
`su?icient number of satellites such that a person with a GPS
`receiver is able to determine their own longitude and latitude
`to within several meters. as well as their elevation. However.
`knowing your own position in longitude and latitude does
`not help others find you without extremely precise topo
`graphical or geophysical maps which also show longitude
`and latitude. Furthermore. the degree of precision in position
`determination is then only accurate to the resolution of the
`maps on hand Nevertheless. the elements for a novel search
`and rescue system. as well as a general purpose locator. are
`made possible by the present invention utilizing GPS tech~
`nology. Before the invention can be explained. however. a
`potential problem with GPS signals must ?rst be explained
`In navigation. a method of guiding ships commonly used
`is dead-reckoning. whereby the known velocity and direc
`tion of travel of a ship from a known position such as a port
`is used to calculate the present position. The drawback is that
`the further a ship moves away from the known position. the
`less accurate the dead-reckoning position becomes. Inclem
`ent weather can further erode the accuracy of a ship’s
`navigation. and endanger lives and property when traveling
`in close proximity to land However. using a GPS receiver
`and a very accurate map with a sut?cient degree of
`resolution. the movements of even a large vessel can be
`guided with a satisfactory degree of precision. The problem
`with GPS signz?s. surprisingly. arises from the high degree
`of precision that the system is able to provide.
`It is the potential application of GPS technology to
`military uses which is responsible for the concern over GPS
`receiver accuracy. Speci?cally. precise positioning of targets
`can enable pinpoint accuracy in the delivery of highly
`destructive military payloads. Therefore. the possibility
`exists that our own satellite network could be used against
`the United States. For this reason. the GPS timing signals
`broadcast by the satellite network for commercial use are
`intentionally made less accurate than the encoded military
`signals. These timing and position errors are called Selective
`Availability (SA) and reduce the accuracy of civilian users
`to roughly 100 meters. While this inaccuracy is irrelevant on
`the high seas. coastal navigation or land-based applications
`such as search and rescue suffer. and potentially destroy the
`bene?ts of GPS technology.
`To overcome the intentional errors introduced in the GPS
`timing signals. a system known as differential GPS (DGPS)
`was developed to reestablish accuracy for civilian users in a
`small. localized area such as coastal navigation. The system
`requires that a permanent GPS receiving and broadcasting
`station be established. and that the precise position of the
`station be determined. Using the fact that the errors intro
`duced by a system of satellites will be the same errors
`transmitted to all receivers in a localized area. a mobile GPS
`receiver in range of the permanent station can determine its
`position and achieve the same degree of accuracy enjoyed
`by the
`This is accomplished by having the perma
`nent station calculate the error introduced by the GPS
`satellites by comparing the signal received with the actual
`known position. This error factor can be transmitted to and
`used by all mobile receivers within the vicinity of the
`permanent station to determine their position accurately to
`within several meters instead of 100 meters. Of course. the
`accuracy of this DGPS determined position decreases the
`further away that a GPS receiver is from the permanent GPS
`receiving and broadcasting station.
`Another form of di?’erential GPS position determination
`has also substantially increased the usefulness of GPS
`receivers. As taught in Smith. US. Pat. No. 5.408.238. a
`comparison of absolute GPS determined locations can be
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`This invention pertains to position determining devices.
`and in particular to devices that enable the position of an
`object or person to be determined relative to another person
`seeking said object. wherein a global positioning system
`receiver is used to determine the distance. direction and
`possible elevation difference between another global posi
`tioning system receiver.
`:
`2. Prior Art
`Being able to determine the precise whereabouts of some
`one or something on or above the surface of the earth has
`long held promise for many purposes. Missing person
`searches would be much simpler if people who were lost had
`a transmitting device with them which constantly broadcast
`their precise position. Such a transmitter would be better
`than just a voice transmitter because the age of the people or
`their medical condition might prevent people from
`responding. or from responding in a helpful manner.
`However. numerous di?iculties arise when actually search
`ing for a transmitter which severely undermines the useful
`ness of such systems.
`For example. US. Pat. No. 4.021.807 teaches how a
`transmitter hidden among stolen money could be used to
`locate those responsible for the theft and the money. A UHF
`homing device hidden among the money is capable of
`transmitting a signal which can be tracked by UHF tracking
`devices. Such a tracking device indicates whether the UHF
`homing signal is being transmitted from the front or rear. and
`from the left or right of a current position and orientation of
`the tracking device. Signal strength can also be used to give
`a crude estimation of distance between the tracking and
`homing devices if the signal is not too distorted by inter
`vening structures.
`The UHF homing signal and tracking devices comprise
`the same principle taught in US. Pat. No. 5.021.794. This
`patent teaches how a miniaturized transceiver carried by a
`child can be remotely activated by a parent to enable the
`child to be located by police cars with UHF trackers.
`One of the drawbacks of such locator systems is that the
`position of the person or object is never known with any
`goat degree of accuracy. Arelated issue is that the reliability
`of the signal received is also suspect. and can not be
`con?rmed. Furthermore. a vehicle with a tracking device
`might circle a homing beacon many times before ?nding it
`due to the crude distance and direction indications of the
`technology.
`Fortunately. a boon to precise location determining
`occurred when the United States saw fit to invest over $12
`Billion in creating a network of 24 satellites in low earth
`orbit. each broadcasting precise timing signals from two
`on-board atomic clocks. Using precise and well-developed
`triangulation and quadrangulation formulas. a receiver that
`picks up signals from several satellites simultaneously can
`determine its position in global coordinates. namely latitude
`and longitude.
`With this network orbiting overhead. a person anywhere
`on the earth has a 24 hour a day line-of-sight view to a
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`ZTE Exhibit 1005 - 10
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`3
`used to determine the relative position or location of the GPS
`devices relative to each other. This comparison eliminates
`the need for a permanent base station which transmits an
`error correction factor because the absolute position of the
`GPS receivers is relevant only so far in that they are
`compared to each other to provide a relative position dif
`ference.
`Returning now to our problem of locating a missing
`person. the exact longitude and latitude provided by DGPS
`is not often useful without very precise maps of su?icient
`resolution and of the area in question. Elevation may also
`play a very important factor if someone is lost in mountain
`ous terrain. Therefore. it would be an advance over the prior
`art if a graphical interface could be provided for a differen
`tial or relative position GPS position detection system which
`would intuitively provide searchers a distance measurement
`and direction. It would also be an advantage if the graphical
`interface provided position information accurate to several
`meters using only GPS signals and positions determined by
`the systems GPS receivers. regardless of whether a perma
`nent station is nearby providing GPS SA error compensation
`information. It would also be an advance over the prior art
`if the difference in elevation between the searchers and the
`lost person could be provided to that same degree of
`accuracy.
`
`25
`
`OBJECTS AND SUMMARY OF THE
`INVENTION
`It is therefore an object of the present invention to provide
`a method and apparatus for locating the relative position of
`a ?rst GPS receiver with respect to a second GPS receiver.
`It is another object to provide a method and apparatus for
`graphically representing the relative position above. such
`that the information is displayed in an intuitive manner.
`It is yet another object of the present invention to provide
`a method and apparatus for determining the difference in
`elevation between the GPS receivers.
`It is still another object to provide a method and apparatus
`for providing the precise distance. direction and elevation to
`a GPS receiver that broadcasts a predetermined signal by
`selectively tuning the apparatus to the signal.
`These and other objects not speci?cally recited are real
`ized in a system of GPS devices which receive civilian GPS
`signals and provide an intuitive graphical interface for
`displaying the relative position of GPS devices in relation to
`each other. the relative position being accurate to several
`meters and de?ned as the distance to. direction of and height
`variance between GPS devices. A ?rst GPS device with the
`person or object to be located transmits its GPS determined
`location to a second GPS device. This second GPS device
`includes a means for receiving the GPS determined position
`of the ?rst GPS device. and also includes means for calcu
`lating the relative position of the ?rst GPS device relative to
`the second GPS device based on a comparison of the
`received telemetry of the ?rst GPS device and its own GPS
`determined position. The relative position of the ?rst device
`is then graphically displayed on an interface of the second
`GPS device in a manner which eliminates the need for a map
`in order to travel to the location of the ?rst GPS device.
`While providing an interface which displays a relative
`position of the ?rst GPS device. this information remains
`accurate no matter how the orientation of the second GPS
`device changes with respect to a compass.
`The system would further include the ability of the second
`GPS device to tune to a signal broadcast by di?erent GPS
`transceiver devices. By selectively tuning to the signal of a
`
`4
`desired GPS device. a distance of. direction to and elevation
`variance of a plurality of di?erent GPS devices is possible.
`Also disclosed is a method for determining the distance.
`direction and elevation to a GPS device. and includes the
`steps of (i) determining a location of a ?rst GPS device
`including a Selective Availability (SA) induced longitude
`and latitude error. (ii) determining a location of a second
`GPS including the approximately same SA induced longi
`tude and latitude error. (iii) transmitting the location of the
`?rst GPS device to the second GPS device. (iv) enabling the
`second GPS device to receive the ?rst GPS device’s telem
`etry signal including the location of the ?rst GPS device. (v)
`comparing the telemetry of the ?rst GPS device to that of the
`second GPS device. and using the comparison of absolute
`longitudes and latitudes to determine a relative distance to.
`direction of and elevation variance between said GPS
`devices. and (vi) displaying the relative position of the ?rst
`GPS device on an interface of the second GPS device in a
`graphical manner so as to intuitively provide the relative
`location of. the distance to and the elevation variance of the
`?rst GPS device relative to the second GPS device.
`DESCRIPTION OF THE DRAWINGS
`FIG. 1 is an illustration of the components in a UHF
`tracking device with the associated position tracking display
`of the prior art.
`FIG. 2A is a perspective view of the components of a
`Global Positioning System (GPS).
`FIG. 2B is an illustration of a GPS receiver and its
`associated display as found in the prior art.
`FIG. 3 is a perspective view of the components of a
`Di?erential GPS (DGPS) system which provides absolute
`longitude and latitude while eliminating the Selective Avail
`ability induced error.
`FIG. 4 is a perspective view of the components in a
`relative GPS system made in accordance with the principles
`of the present invention.
`FIG. 5A is the preferred embodiment of an interface
`providing a graphical display for the relative position deter
`mining GPS device system illustrated in FIG. 4.
`FIG. 5B is a variation of the preferred embodiment shown
`in FIG. 5A.
`FIG. 5C shows how the arrow of a graphical display
`remains stationary relative to a ?xed reference point (a
`compass) when the GPS device is rotated relative to the
`compass.
`FIG. 5D illustrates a modi?cation to the preferred graphi
`cal display embodiment of FIG. 5A.
`FIG. 6 is an alternative embodiment of an interface
`providing a graphical display for the system of GPS devices
`illustrated in FIG. 4.
`FIG. 7A is an alternative embodiment of an interface
`providing a graphical display of variance in elevation for the
`system of GPS devices illustrated in FIG. 4.
`FIG. 7B is a variation of the embodiment of FIG. 7A.
`FIG. 8 is a block diagram of the components of the
`relative GPS system used in FIG. 4.
`FIG. 9 is a perspective view of another embodiment of the
`present invention.
`FIG. 10 is a perspective view of another embodiment of
`the present invention.
`DETAILED DESCRIPTION OF THE
`INVENTION
`FIG. 1 illustrates the components and a typical display of
`a UHF tracking system. As shown. a transmitter 10 is at
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`ZTE Exhibit 1005 - 11
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`some unknown location some distance from the tracking
`device 20. The tracking device is typically mounted inside a
`vehicle. such as a police car. When the transmitter 10 is
`activated. the tracking device “homes in" on the transmitter.
`This is accomplished by a display 30 indicating whether the
`transmitter 10 is in front 40 or in back 50. to the left 60 or
`the right 70 of the tracking device 20. A distance indicator
`75 also shows a relative distance to the transmitter 10 by
`indicating the strength of the signal received.
`Such a system only provides vague references to the
`location of the transmitter 10 at best. For example. the
`direction of the transmitter 10 can only be known to within
`90 degrees. This is because the front\back and left\right
`indicators 40. 50. 60 and 70 only de?ne four quadrants. 80.
`82. 84 and 86 in which the transmitter 10 can be found. In
`addition. because the distance indicator 75 relies only on a
`measure of the signal strength received. distortion or inter
`ference with the transmitted signal can give a false indica
`tion of actual distance to the transmitter 10. There is also no
`way to know whether there is interference until a UHF
`transmitter 10 is tracked down. Furthermore. the UHF signal
`tracker 20 cannot indicate a height variance between the
`transmitter 10 and the tracking device 20. A tracker using a
`UHF signal tracker 20 mounted in a car might arrive at a
`mountain and still show substantial distance to the trans
`mitter l0. and yet the distance might be vertical and impass
`able. Forewarning of great altitude variations is helpful in
`planning the method and supplies required for tracking.
`FIG. 2A illustrates the original concept of the Global
`Positioning System (GPS). A GPS receiver 100 receives
`timing signals from at least three. and preferably four low
`earth orbiting satellites 110. 120. 130 and 140. The timing
`signals are provided by extremely accurate atomic clocks in
`the satellites. two redundant clocks aboard each satellite
`providing backup. Three satellites provide sufficient infor
`mation for a GPS receiver 100 to calculate a longitude and
`latitude using triangulation formulas well known to those
`skilled in the art. If a signal can be received from four
`satellites. the altitude of the GPS receiver 100 can also be
`determined using a modi?ed formula.
`FIG. 2B illustrates a typical display of a GPS device 100
`as found in the prior art which provides location information
`to the user in longitude 142 and latitude 144 coordinates.
`This is because the GPS was originally intended for use as
`an absolute location determining device and had only an
`antenna 146 for receiving GPS signals. In this con?guration.
`the only useful information the GPS device can provide is
`coordinates which can be used to ?nd a location on a map.
`FIG. 3 illustrates the differential GPS (DGPS) concept
`that was made necessary by the military’s introduction of an
`error into the GPS signals broadcast by the GPS satellites.
`For coastal navigation. a series of permanent GPS stations
`200 such as the one shown broadcast an error correction
`code which enables mobile GPS receivers 210 in the vicinity
`of the permanent GPS station 200 to determine their location
`to the same level of accuracy enjoyed by military systems.
`The Selective Availability (SA) error is corrected by using
`the previously determined accurate location of the perma
`nent station 200. receiving the GPS signals to calculate a
`location. determining the error between the broadcast posi
`tion and the known position. and then broadcasting the error
`correction factor to mobile GPS receivers. GPS receivers
`210 then correct their own GPS calculated position using the
`broadcast correction factor. The error correction factor is
`thus only accurate for GPS receivers near the permanent
`station.
`While the DGPS system does restore accuracy to the GPS
`location calculations. the system is only useful for search
`and rescue or location determination if very detailed maps
`are available.
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`FIG. 4 illustrates the preferred embodiment of the present
`invention which overcomes the need for detailed maps when
`locating a GPS receiver made in accordance with the prin
`ciples of the present invention. The same number of satel
`lites are necessary as in the normal GPS position determin
`ing system of FIG. 1. Three satellites 300. 310 and 320
`provide su?icient information to determine a position. and a
`fourth satellite 330 can provide altitude information. What
`should also be explained before discussing the operation of
`the GPS devices of the present invention is that while the
`term “receiver” is accurate or GPS device of the prior art. the
`GPS devices of the present invention can be receivers or
`transceivers. depending upon the particular application of
`the present invention. Therefore. the speci?cation will now
`refer to GPS devices which implies that they can be either
`receivers or transceivers. A last convention to note is that the
`“?rst GPS device" is always assumed to be the GPS device
`being tracked. and the “second GPS device" will always be
`assumed to be the GPS device which is receiving telemetry
`so as to track the ?rst GPS device. unless otherwise noted.
`As stated previously. the diiferential or relative location
`determining method used in the present invention is different
`from that described in FIG. 3. This method eliminates the
`need for permanent GPS stations which provide error
`correction. because the location of the GPS device de?ned
`by the actual longitude and latitude is relevant only insofar
`as they are used to calculate the distance between a ?rst or
`tracked GPS device and a second or tracking GPS device.
`The only limitation is that the induced SA error be nearly the
`same for both receivers to achieve a distance calculation
`accurate to less than 100 meters. This requirement is easily
`satis?ed because the induced SA position error will be
`nearly the same for GPS devices within one hundred miles
`of each other and therefore substantially insigni?cant. In
`addition. as the GPS receivers get closer. the error becomes
`negligible. What should be obvious. therefore. is that dis
`tance is always accurate to at lease 100 meters.
`The ?rst and second GPS devices are capable of deter
`mining their location in terms of longitude and latitude
`according to the methods well known to those skilled in the
`art through triangulation (location) and quadrangulation
`(location and elevation) formulas. The innovation of the
`present invention begins with the ?rst GPS device 340 being
`modi?ed to be a transceiver so as to transmit this location or
`location and elevation as telemetry data. Another point of
`novelty is that the second GPS device 350 is modi?ed not
`only to receive GPS signals. but also to receive this telem
`etry data from the ?rst GPS receiver.
`A furthm' modi?cation is that the second GPS device 350
`is advantageously and selectively tuneable to receive telem
`etry from a desired frequency. This enables the second GPS
`device 350 to be be able to track multiple GPS devices. It is
`also possible to provide a tuner such that a plurality of GPS
`devices can be simultaneously tracked and displayed on the
`second GPS device 350 interface. These features also imply
`that the ?rst GPS device 340 can advantageously selectively
`transmit telemetry on a desired frequency.
`After receiving the telemetry transmission of the ?rst GPS
`device 340. device 350 calculates a relative distance
`between the GPS receivers 340 and 350 by comparing
`absolute longitudes and latitudes. The interface of the sec
`ond GPS device 350 then graphically displays the position
`of the ?rst GPS device 340 relative to the second GPS device
`350 in an intuitive manner which facilitates immediate travel
`to the ?rst GPS device 340 without consulting a map.
`Speci?cally. the interface 352 of the second GPS receiver is
`shown in FIG. 5A and is comprised of an LCD screen 352.
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`ZTE Exhibit 1005 - 12
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`5.781.150
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`7
`such as the type used in portable notebook computers but
`smaller. The interface 352 consists of an arrow 354. an end
`356 of the arrow 354 generally ?xed on the display 352 and
`an opposite pointing end 358 of the arrow 354 which
`continuously points in the direction of the ?rst GPS device
`340. This is accomplished by pivoting or rotating the arrow
`354 about the ?xed end 356. The circle 36!) de?nes the limit
`of travel of the arrow 354 on the interface 352 and does not
`need to be shown. However. if left on the display. the circle
`360 can be conveniently divided by tick marks 362. as
`shown in close-up view FIG. 5B. The tick marks 362
`represent the 360 degrees of a compass.
`Returing now to the system of GPS devices. the second
`GPS device 350 is constantly receiving updated telemetry
`data from the ?rst GPS device 340 and from the GPS
`satellites 300. 310. 320. 330 overhead This allows the
`second GPS device 350 to continuously update the direction
`in which the arrow 354 is pointing. This ability is crucial
`because the orientation of the second GPS device 350
`relative to a compass may be changing constantly.
`Therefore. the present invention envisions that a user will be
`able to hold the second GPS device 350 and turn in a circle.
`and the arrow 354 will always point toward the ?rst GPS
`device 340. This implies that the circle 360. if shown. also
`remains ?xed relative to the compass. This ability is a result
`of an internal compass of the second GPS device 350. The
`internal compass provides a ?xed reference point relative to
`which the continuously displayed arrow 354 will use to
`always point toward the ?rst GPS device 340.
`The feature described above is illustrated. for example. in
`FIG. 5C. For this drawing. the direction north of the ?xed
`compass 368 is toward the top of the paper. The direction
`“north” might be true north or magnetic north. The two GPS
`devices illustrated are the same GPS device 366. but shown
`in two different positions or orientations relative to the ?xed
`compass 368. What remains constant (as long as the object
`being tracked does not move) is that the arrow 354 always
`points due east to some tracked GPS device whose telemetry
`data has been received by the pictured GPS device 366. Not
`shown because of the scale of the drawing is the fact that the
`arrow 354 also points to the same tick mark 362 at approxi
`mately 90 degrees. the circle 360 and tick marks 362 also
`remain ?xed relative to the compass 368.
`With respect to the intuitive nature of this preferred
`embodiment shown in FIG. 5A. it should be noted that while
`the direction to travel is displayed graphically on this
`particular display. distance is not. Distance. as well as other
`useful but presently nongraphically displayed information is
`displayed as text in an unused portion of the LCD screen
`352. This information includes but is not limited to the
`selected telemetry frequency or frequencies of remote ?rst
`GPS devices 340. It is also possible to choose a units of
`distance for the displayed distance measurement shown as
`text so as to conform to user preferences for the U.S. or
`metric system.
`While the preferred embodiment has discussed a ?rst GPS
`device 350 which does not receive but only transmits
`telemetry data. and a second GPS device 350 which does the
`reverse. it should be obvious that the second GPS device 350
`can be modi?ed to transmit as well as to receive telemetry
`data. and that more than one of these modi?ed second GPS
`350 type devices can be used. This enables the users of a
`system of two second GPS type devices 350 to simulta
`neously move toward each other as depicted in FIG. 10.
`A variation of the arrow 354 with an end 356 ?xed at a
`center of a circle 360 representing the location of the second
`
`8
`GPS device 350 is an arrow 370 as shown in ?gure SD.
`Instead of being anchored at an end point 356. this arrow 370
`rotates about a midpoint of the arrow 370. The advantage of
`this design is that it provides a larger arrow 370 within the
`relatively small LCD display screen 352 of the second GPS
`device 350.
`FIG. 6 illustrates an alternative embodiment of the graphi
`cal screen display of FIGS. 5A and SD. The displayed
`information can be modi?ed to present different and advan
`tageously more useful and intuitive information to the user.
`at a cost to the user of more circuitry and sophistication of
`the GPS devices. More intuitively useful information is
`displayed on the interface 352 by replacing the direction
`arrows 354 or 370 with a grid. Centered on the location of
`the user or second GPS device 350. represented by some
`type of mark 372. are a plurality of increasingly larger
`concentric circles 374. The circles 374 are scaled so as to
`represent uniformly spaced distances. Finally. some type of
`mark 378 such as a small circle. square or other designation
`which is easily visible on the screen represents the ?rst GPS
`20
`I device 340 which is being tracked.
`The signi?cant advantage of this display is that not only
`does it show the direction to travel. but at a single glance
`gives the user some easily discerni